1. Technical Field
The present invention relates to output drivers in general, and, in particular, to an output driver having electrostatic discharge protection.
2. Description of Related Art
An electrostatic discharge (ESD) event is defined as a transfer of charges between bodies of different electrostatic potentials in proximity or via direct contact. ESD poses a reliability concern for integrated circuit devices. Different ESD models, such as human body model (HBM), machine model (MM) and charged device model (CDM), have been used for testing integrated circuit devices to make sure they are adequately protected against an ESD event. The difference among various ESD models mainly lies upon the amount of current delivered to an integrated circuit device to emulate an ESD event encountered by the integrated circuit device.
From a circuit design standpoint, ESD clamps are typically utilized to protect an integrated circuit device against an ESD event. An ESD clamp is effectively a large switch that is normally turned off except in the presence of an ESD event. During an ESD event, the switch is turned on to produce a conductive path for charges to be drained into a grounded network within the integrated circuit device.
Output drivers of an integrated circuit device are generally susceptible to ESD events because they are commonly connected to external pins that can be contacted by either human beings or machines. Thus, an ESD clamp is typically employed within an output driver to provide ESD protection. On the one hand, it is imperative to have an ESD clamp that is large enough to provide the maximum ESD protection. But on the other hand, the ESD clamp cannot be too large because it is costly and may affect the performance of the output driver.
The present disclosure targets the problem of placing an ESD clamp in an output driver such that the output driver meets the performance requirements as specified by technology developers while without compromising ESD protection.